Magnetic (or magneto-resistive) random access memory (MRAM) is a non-volatile access memory technology that could potentially replace the dynamic random access memory (DRAM) as the standard memory for computing devices. Particularly, the use of MRAM-devices as a non-volatile RAM will eventually allow for “instant on”-systems that come to life as soon as the computer system is turned on, thus saving the amount of time needed for a conventional computer to transfer boot data from a hard disk drive to volatile DRAM during system power up.
A magnetic memory element (also referred to as a tunneling magneto-resistive or TMR-device) includes a structure having ferromagnetic layers separated by a non-magnetic layer (barrier) and arranged into a magnetic tunnel junction (MTJ). Digital information is stored and represented in the magnetic memory element as directions of magnetization vectors in the ferromagnetic layers. More specifically, the magnetic moment of one ferromagnetic layer is magnetically fixed or pinned (also referred to as a “reference layer”), while the magnetic moment of the other ferromagnetic layer (also referred to as “free layer”) is free to be switched between the same and opposite directions with respect to the fixed magnetization direction of the reference layer. The orientations of the magnetic moment of the free layer are also known as “parallel” and “anti-parallel” states, respectively, wherein a parallel state refers to the same magnetic alignment of the free and reference layers, while an anti-parallel state refers to opposing magnetic alignments therebetween.
Depending upon the magnetic states of the free layer (i.e., parallel or anti-parallel states), the magnetic memory element exhibits two different resistance values in response to a voltage applied across the magnetic tunnel junction barrier. The particular resistance of the TMR-device thus reflects the magnetization state of the free layer, wherein its resistance is “low” when the magnetization is parallel, and “high” when the magnetization is anti-parallel. Accordingly, a detection of changes in resistance allows a MRAM-device to provide information stored in the magnetic memory element, that is to say to read information from the magnetic memory element. In addition, a magnetic memory element is written to through the application of a bi-directional current in a particular direction, in order to magnetically align the free layer in a parallel or anti-parallel state.
An MRAM-device integrates a plurality of magnetic memory elements and other circuits, such as a control circuit for magnetic memory elements, comparators for detecting states in a magnetic memory element, input/output circuits and miscellaneous support circuitry. As such, there are certain microfabrication processing difficulties to be overcome before high capacity/density MRAM-devices become commercially available. For example, in order to reduce the power consumption of the MRAM-device and provide a variety of support functions CMOS-technology is required. As is known in the art, various CMOS processing steps are carried out at relatively high temperatures, while ferromagnetic materials employed in the fabrication of MRAM-devices require substantially lower process temperatures. Thus, the magnetic memory elements typically are designed to be integrated into the back end wiring structure of back-end-of-line (BEOL) CMOS processing following front-end-of-line (FEOL) CMOS processing.
To be useful in present day electronic devices, very high density arrays of magnetic memory cells are utilized in magnetic random access memories. In these high density arrays the magnetic cells are generally arranged in rows and columns, with individual cells being addressable for reading and writing operations by the selection of an appropriate row and column containing the desired cell. Also, conveniently, orthogonal current lines are provided, one for each row and one for each column so that a selected cell is written by applying current to the appropriate row current line and the appropriate column current line.
Recently, and especially in view of modern portable equipment, such as portable computers, digital still cameras and the like, the demand of low-cost and particularly high-density mass storage memories has increased dramatically. Therefore, one of the most important issues for low-cost and high-density MRAM-devices is a reduction of the MRAM-cell size. However, down-scaling MRAM-cells requires smaller and smaller magnetic tunnel junctions and, therefore, a lot of severe problems arise, since for a given aspect ratio and free layer thickness the activation energy, being dependent on the free layer volume, scales down like w, where w is the width of the magnetic cell. Otherwise, the switching fields increase roughly like 1/w. Thus, in scaling down a MRAM-cell field selected switching becomes ever harder, but at the same time the magnetic cell looses its information more and more rapidly due to thermal activation.
In light of the above, there is a need to provide a magnetic memory element and magnetic random access memory (MRAM) device comprising such magnetic memory elements enabling a cell-size down-scale without thereby causing severe problems with respect to an increase of switching-fields and decrease of activation energy.